1. Field of the Invention
This invention relates to a furnace for providing heated circulation air to an interior comfort space, and is more particularly directed to a condensate trap for a gas fired forced air furnace of the type having a heat exchanger.
2. Description of the Prior Art
In conventional gas-fired forced air furnaces a thermostat senses the temperature in the comfort zone relative to a predetermined set point temperature. When the temperature is below the set point, the thermostat closes to supply thermostat ac power to the furnace as a call for heat. This initiates a sequence of events that ultimately causes the furnace to come on. An inducer motor is enabled to flow combustion air across the burners, and through a condensing heat exchanger, after which a gas valve is actuated to supply gas to the gas burners. An ignition device is also actuated to light the burners. A flame sensor then proves burner ignition and sends power to a burner delay timer. Then after a predetermined blower delay time, which varies with furnace design, the furnace blower is actuated. The blower moves circulating room air from a return air duct through the furnace heat exchanger to pick up heat from the heated combustion products (carbon dioxide and water vapor) from the gas burners. The heated circulate air then goes into a hot air plenum and is distributed through hot air ductwork back to the comfort space. When the comfort space air is warmed sufficient to reach the thermostat set point, the thermostat terminates the call for heat. When this happens the blower and burners go through a shut off sequence and the furnace awaits the next call for heat.
The present invention mainly deals with handling of large amounts of condensate that form in the condensing heat exchanger and also in the inducer housing. When the inducer motor is in operation a substantial step-up in pressure occurs between the condensing heat-exchanger and intake of the inducer housing on the one hand, and the outflow of the inducer housing on the other hand. Typically there is negative pressure (relative to atmospheric pressure) at the intake, and positive pressure at the outflow. In the prior art a trap 110, shown in FIG. 5, and having a single chamber has been used to simultaneously trap condensate forming in both pressure compartments. In this trap a first tube 112, in fluid communication with a source of high pressure, leads downward to terminate beneath the surface of a reservoir 117 of water. An overflow drain 115 is situated a distance A above the terminus of the tube. A second tube 114, connected to a source of negative pressure, also has a terminus beneath the surface of the reservoir. Condensate forming within the sources drips or flows through the tube 112, 114 into the reservoir 117. The overflow drain is located a distance B beneath the top of the chamber. In operation the water levels 116, 118 within the tubes 112, 114 are displaced toward and away from the termini of the tubes in accordance with the pressure heads in their respective sources. It is evident that the distance A is a measure of the amount of positive pressure that can be withstood by the trap before gases from the positive source are forced through the end of the tube to bubble into the chamber. Also distance B is a measure of the amount of negative pressure the trap can handle before the reservoir contents are sucked back into the negative source. Thus the sum A+B is a lower limit on the vertical dimension of this single-chambered trap.
Older furnaces typically create positive pressure of about 11/2 inches (3.8 cm) of water, and negative pressure of about 21/2 inches (6.3 cm) of water, so that the trap only had to be 4 inches (10.2 cm) tall. Modern high flow furnaces can produce about 4 inches (10.2 cm) of positive pressure and 4 inches (10.2 cm) of negative pressure, requiring the trap to be at least 8 inches (20.3 cm) tall.
Also, on older furnaces pressure differentials within the inducer housing due to rotation of the inducer wheel or fan impaired the drainage of condensate from the housing. Condensate would then build up in the housing and eventually hinder the inducer's ability to move flue gases through the furnace.